It is known, as for example from commonly owned U.S. Pat. No. 3,926,706 to produce synthetic-resin sheeting by extruding a thin-wall tube that is internally inflated to cool and stretch it, and then flattening this tube. Two concentric tubes can be thus formed with different degrees of stretch to form a laminated sheet. The wall thickness of the film must be as uniform as possible. Typically a variation in thickness of +/-8% is the maximum acceptable, and preferably it should be at most +/-5%.
To control this thickness, which depends mainly from the viscosity of the resin and the size of the passage it is being extruded through, it is known to differentially heat and cool portions of the tube in the extruder head. Thus, as described in German patent document No. 2,947,293, an annular array of heat-exchange compartments is provided around the extrusion nozzle. Each heat-exchange compartment is associated with a respective sensor and source of heat-exchange liquid so that it can heat the respective section of the tube being extruded when it is too thick, thereby making it less viscous, or cool the section when it is too thin.
This arrangement is extremely complex. For best control of the tube thickness it is necessary to provide at least one hundred such compartments and sensors. The control system for these elements is extremely unwieldy and prone to breakdown. Even when it works perfectly, this system rarely produces a satisfactorily uniform product.
Other systems are known which have a plurality of angularly equispaced and radially extending adjustment screws that can be adjusted to control the passage thickness. Such arrangements are extremely hard to operate and cannot normally be adjusted while a tube is being formed. Much less it is impossible for them to compensate for momentary problems, as for instance when a lump of inadequately plastified resin comes into the passage and blocks it at one location.